Cyanoacrylate monomer adhesives are well known and widely used as "instant adhesives", so-called because of their very rapid polymerization on contact with surfaces containing even very weak anion sources. However, a notorious problem of cyanoacrylate polymers is their susceptibility to thermal degradation at only moderately high temperatures. As a consequence, the instant adhesive benefits of cyanoacrylate monomers have not been available in many applications where the bonded substrates may be subjected to intermittent temperatures in excess of 120.degree. C. or extended exposure to temperatures of about 80.degree. C. This problem of thermal stability of cyanoacrylate polymers is distinct from the problem of stabilizing monomer formulations against premature polymerization. However, to preserve the instant adhesive benefits of cyanoacrylates, it must be appreciated that improvements in polymer thermal stability should not significantly degrade the storage stability or cure speed of the monomer formulation from which the polymer is derived.
Numerous attempts have been made to improve the thermal stability of cyanoacrylate adhesive bonds. In U.S. Pat. No. 3,832,334, the addition of maleic anhydride is said to produce adhesives which have increased thermal resistance while preserving fast cure speed of the cyanoacrylate adhesive.
In U.S. Pat. No. 4,196,271, tri-, tetra- and higher carboxylic acids or there anhydrides are said to be useful form improving heat resistance of cured cyanoacrylate adhesives. Phthalic anhydride is reported to improve heat resistance of cyanoacrylate adhesive bonds in U.S. Pat. No. 4,450,265 and benzopheonetetracarboxylic acid or its anhydride are reported to provide a superior heat resistance for cyanoacrylate adhesives in U.S. Pat. No. 4,532,293.
According to Chem. Abst., 85:64138p a cyanoacrylate adhesive which includes a graft copolymer of methyl methacrylate and a fluorine containing rubber as a plasticizer is reported to give improved stability to thermal shocks. Cyanoacrylate adhesives containing elastomeric acrylic rubbers are reported to give improved properties, particularly after exposure to elevated temperatures, in U.S. Pat. No. 4,440,910.
In U.S. Pat. No. 4,490,515, cyanoacrylate compounds containing certain maleimide or nadimide compounds are reported to improve the hot strength properties of cyanoacrylate adhesives.
Mixtures of certain sulfone compounds and a dicarboxylic acid or dicarboxylic anhydride are said to greatly improve heat resistance of cyanoacrylate adhesives in JP 55/066980.
In Chem. Abst., 80 (22): 121806c (abstracting) JP 48/8732, cyanoacrylates containing 3-25% divinyl sulfone are reported to have improved heat resistance.
U.S. Pat. No. 4,560,723 describes certain cyanoacrylate adhesives containing a certain treated copolymer toughener, and a "sustainer" compound having certain compatibility properties said to provide improved retention of toughness on heat aging of the cured adhesive. Various substituted aryl compounds are identified as suitable "sustainers," including nitrobenzene, 1,2-dichlorobenzene, 1,2,4-trichlorobenzene and bromochlorobenzene.
Despite this extensive work in the art, there continues to be a need to identify materials which will improve the heat performance of cyanoacrylate adhesives so as to increase the options available to the formulator and/or provide further improvements over the existing additives known to enhance thermal resistance of cyanoacrylate polymers.
Various sultone compounds, including propane sultone, bromo phenol blue, 1,8-naphthosultone, butane sultone and 3,3',5,5'-tetramethylbutane sultone have been disclosed to be useful as shelf-life stabilizers and, at levels greater than 2%, as bond plasticizers for cyanoacrylate adhesives in U.S. Pat. 3,742,018, GB 1,196,069 and BE 353,262. It has subsequently become known, however, that the stabilizing activity of such sultones is inconsistent, the system requiring a minimal level of water contamination in the cyanoacrylate monomer in order to achieve effective shelf-life stabilization. The stabilization activity of sultone compounds has thus been traced to the sulfonic acid hydrolysis product rather to the sultone itself and commercial use of sultones as cyanoacrylate shelf-life stabilizers has been relegated to the status of a chemical intermediate. Only readily hydrolyzed sultones are utilized and hydrolysis of the sultone is undertaken prior to addition to the monomer so as to obtain consistent results regardless of water contamination level. No commercial use of sultones as bond plasticizers is known to have developed.